Electron beam image recording applications include, for example, automated cartography (see "The Versatility of Electron Beam Techniques for Image Recording", U.S.A.F. Symposium on Image Display and Recording, p. 273, April 1969, "Electron Beam Image Recording Applications", ELIM'S-70 Symposium, p. 199, April 1970, and "Investigations of the Use of Conventional Films In the ETL Cartographic EBR, Government Report ETL-0177, Mar. 15, 1979) and involve the direct imaging of silver halide photographic emulsions with high energy (e.g., 15 KeV) electrons. Such imaging techniques afford the potential of very high resolution due to the short effective wavelength and high productivity due to independent x and y positioning.
Silver halide emulsions suitable for use in an electron beam recording process are well known and are described, for example, in U.S. Pat. No. 3,428,451, issued Feb. 18, 1969, and U.S. Pat. No. 4,837,135, issued Jun. 6, 1989 and references cited therein.
During the imaging process, impingement of electrons on the imaging media generates a space charge within the media due to both capture of the imaging electrons and to hole generation which arises as a result of secondary electron emission. With electrons in the 15 to 20 KeV range, the space charge that results is predominately negative in sign. This negative charge generation and the resulting repulsion of the imaging electrons can lead to such problems as geometric or positional image distortions, spurious changes in image resolution, and variations in optical density of the recorded image unless an adequate ground plane is maintained in close proximity to the space charge. Ideally, this ground plane is provided by a conductive layer incorporated within the imaging media between the film support material and the imaging layer. The maximum resistivity of this conductive layer is in part a function of the path length to ground and the grounding mechanism. For grounding at the edge of narrow width film, i.e., short path lengths, resistivities less than about 5.times.10.sup.8 .OMEGA./sq are required. Longer path lengths require even lower resistivities.
Electron beam recording film images are typically used as originals for the generation of secondary images, e.g., lithographic plates and cartographic prints, and, therefore, must have a low processed D.sub.min in both the UV and visible wavelength and must exhibit a high degree of uniformity. A UV D.sub.min of no greater than 0.12 density units, preferably no greater than 0.10 is needed. The uniformity of the UV density across the film is preferably at least within .+-.0.02. A visible D.sub.min of no greater than 0.07, preferably no greater than 0.04, is also required.
A variety of materials have been described for use in conductive layers on conventional photographic films. Typically, the conductivity of these layers is sufficient to provide antistatic protection that helps minimize problems such as static marking and dirt and dust attraction that may otherwise result from triboelectric charging of photographic films during manufacture and use. To provide antistatic protection to the photographic films the resistivities of these conductive layers need to be less than about 10.sup.11 .OMEGA./sq. Antistatic layers comprising ionically conductive materials such as inorganic salts, colloidal silicas, polymeric salts such as sulfonic acid salt homopolymers and interpolymers are well known in the art. However, the electron beam imaging process requires that the actual imaging be done at very high vacuum, thus, ionically conductive materials that require the presence of moisture to solvate the conductive species are incapable of providing the required resistivity values under the high vacuum, extremly low humidity conditions of the imaging process.
Electronically conductive materials such as semiconductive metal salts, for example, cuprous iodide, described in U.S. Pat. Nos. 3,245,833, 3,428,451 and 5,075,171 reportedly provide resistivities less than 10.sup.7 .OMEGA./sq. However, these conductive layers have high UV densities and are typically applied from harmful solvents such as acetonitrile which also makes them undesirable from a health and environmental standpoint. In addition, these cuprous iodide/acetonitrile coating compositions lead to conductive layers that exhibit a "mottled" appearance.
Conductive layers comprising inherently conductive polymers such as polyacetylene, polyaniline, polythiophene, and polypyrrole are described in U.S. Pat. No. 4,237,194, JP A2282245, and JP A2282248, but these layers are highly colored.
Conductive fine particles of crystalline metal oxides dispersed with a polymeric binder have been used to prepare humidity-insensitive, conductive layers for various imaging applications. Many different metal oxides are alleged to be useful as antistatic agents in photographic elements or as conductive agents in electrographic elements in such patents as U.S. Pat. Nos. 4,275,103, 4,394,441, 4,416,963, 4,418,141, 4,431,764, 4,495,276, 4,571,361 and 4,999,276. Preferred metal oxides are antimony doped tin oxide, aluminum doped zinc oxide, and niobium doped titanium oxide. However, these materials do not provide acceptable performance characteristics in the demanding application of the present invention. In order to obtain high electrical conductivity, a large amount (100-10000 mg/m.sup.2) of metal oxide must be included in the conductive layer. This results in decreased transparency for thick conductive coatings. The high volume fraction of the conductive fine particle in the conductive coating needed to achieve high conductivity also results in brittle films subject to cracking and poor adherence to the support material.
Fibrous conductive powders comprising, for example, antimony doped tin oxide coated onto non-conductive potassium titanate whiskers have been used to prepare conductive layers for photographic and electrographic applications. Such materials have been disclosed in U.S. Pat. Nos. 4,845,369, 5,116,666, JP A-63098656 and JP A-63060452. Layers containing these conductive whiskers dispersed in a binder reportedly provide improved conductivity at lower volume fractions than the aforementioned conductive fine particles as a result of their higher aspect (length to diameter) ratio. However, the benefits obtained as a result of the reduced volume fraction requirements are offset by the fact that these materials are large in size (10 to 20 .mu.m long and 0.2-0.5 .mu.m diameter). The large size results in increased light scattering and hazy coatings. Reducing the size of these particles by various milling methods well known in the art in order to minimize light scattering is not feasible since the milling process erodes the conductive coating and therefore degrades the conductivity of these powders.
Transparent, binderless, electrically semiconductive metal oxide thin films formed by oxidation of thin metal films which have been vapor deposited onto film base are described in U.S. Pat. No. 4,078,935. The resistivity of such conductive thin films has been reported to be 10.sup.5 .OMEGA./sq. However, these metal oxide thin films are unsuitable for electron beam imaging applications since the overall process used to prepare them is complex and expensive and adhesion of these thin films to the film base and overlying layers is poor.
It is toward the objective of providing an improved electron beam imaging film that is free of objectionable visual density, UV density, and mottle, and can be manufactured without the need for organic solvents such as acetonitrile that the present invention is directed.